1. Field of Endeavor
The present invention relates to wells and cementing and more particularly to cementing a wellbore using cementing material encapsulated in a shell.
2. State of Technology
Cement is used to seal the annulus between the outer wall of a drilled hole, and the casing that is placed inside the hole. Obtaining a complete and strong seal in all portions of the drilled hole is challenged by a number of factors, prominently including the fact that the annulus is an irregular shape with occasional areas that are much larger than the average, or that intersect fractures and open spaces in the original formation. Compounding this is the problem that in order to emplace the cement, all of the drilling fluid (referred to as mud because of the high content of mineral additives and rock cuttings) must be removed from the annular space. This is typically done by displacing the mud with the cement compound, placing significant restrictions on the properties of the liquid cement. Any locations in which the drilling mud is not completely replaced by cement represent weak areas in the completion. This type of weakness is a known location for well failures such as that of the Macondo well in the Gulf of Mexico, where the Deepwater Horizon explosion and spill occurred in 2010.
A very large number of methods have been evolved for effective placement of cement, including the use of polymer-based cements that are activated by radiation. These can be either derived directly from a polymer-based drilling mud, or can be separate cementing agents that utilize intense radiation to initiate the hardening of the cement for a variety of timing and temperature/chemistry issues. These types of cements are described in U.S. Published Patent Application 2011/0265996 by Halliburton Law Department titled “Radiation-Induced Thickening for Set-on-Command Sealant Compositions and Methods of Use.” Limitations associated with the use of radiation-induced polymer cements have not been widely documented, but the use of intense radiation sources as the setting mechanism is a strong limitation to their widespread use.
A closely related field is that of polymers for blocking permeability in fractures and high permeability zones in wells (thief zones). A considerable literature exists on the use of polymers to block these zones, generally by “gelling” in place. That formation of gel is a gradual cross linking of the polymer leading to a strong, cement-like material. However, since this material must be pumped as a liquid from the surface while it progressively gels over time, it is generally acceptable to have it form a weak cement in order to ensure that it does not set too quickly and clog the pumping equipment or well bore. The materials used for this purpose, however, appear to be usable to make stronger cements if the need to pump them quickly while still a liquid were avoided. Discussion of this patent literature can be found in U.S. Pat. No. 5,079,278 to Mobile Oil Corporation for “Enhanced Oil Recovery Profile Control with Crosslinked Anionic Acrylamide Copolymer Gels.”